The present invention relates to filter devices used in the microwave region, and particularly to improvements of microwave filter devices consisting of strip lines.
In circuit systems processing microwave signals, filter devices such as band-pass filters (BPF) or band-rejection filters (BRF) are important circuit elements. Along with the recent trend aiming toward lighter-weight, smaller-size electronic equipment, there has been an enhanced need in particular for filter devices using conductor lines and strip lines which can be mounted on the same circuit board with the circuit parts.
Various types of microwave filter devices using such strip lines have been developed in the past. Typically, they can be summarized in those illustrated in FIGS. 1-5.
The microwave filter device 10 illustrated in FIG. 1 is generally called a half-wave distributed-coupling type band-pass filter (BPF). Between its input line 11 and output line 12 it has resonator lines 13, 14 arranged next to each other on two-dimensional planes, displaced from each other by the equivalent of a quarter wavelength, and separated from each other by distributed coupling gaps 15, 16, 17.
The microwave filter device 20 illustrated in FIG. 2, although of the same distributed-coupling type, is a BPF device of the type called the quarter-wavelength type. The resonator lines 23, 24 are arranged next to each other between the input line 21 and output line 22 and are separated from each other by distributed coupling gaps 25, 26, 27. Their lengths are both set to equal a quarter wavelength, and they are each grounded at the opposite ends. This BPF device 20 is also called the interdigital type.
The microwave filter device 30 illustrated in FIG. 3 is a BPF device of the type called the tip-coupled type. One end of the first resonator line 33, which has a length equivalent to a half wavelength, faces towards the tip of the input line 31, from which it is separated by a gap 35 for capacitive coupling. One end of the second resonator line 34, which also has a length equivalent to a half wavelength, faces towards the other end of the first resonator line 33, from which it is likewise separated by a gap 36 for capacitive coupling; and one end of the output line faces towards the other end of the second resonator line 34, similarly separated from it by a gap 37 for capacitive coupling.
Unlike the distributed-coupling type in which the lines are separated by a number of gaps, as described above, there are microwave filter devices using quarter-wavelength lines such as those shown in FIGS. 4 and 5 which are composed of strip lines which are connected completely ohmically with respect to direct current.
The one shown in FIG. 4 is a microwave band-pass filter device 40 of a type called the quarter-wavelength line-coupled type. Its resonator lines 45, 46, 47, each having a length equivalent to a half wavelength, are arranged orthogonally towards its input and output lines 41, 42 and towards the connecting parts between them, which are coupling lines 43, 44 each having a length equivalent to a quarter wavelength.
The microwave filter device 50 shown in FIG. 5 is a band-rejection filter (BRF) device in which the resonator lines 54, 55, which each have lengths equivalent to a quarter wavelength, are formed orthogonally towards the coupling line 53, which couples the input and output lines 51, 52 and has a length equivalent to a quarter wavelength, and towards the connecting parts with each of the input and output lines.
These conventional microwave filter devices 10-50 are each configured on two-dimensional planes on dielectric substrates. In most cases, the dielectric substrates are printed circuit boards which mount the filter devices together with their peripheral circuits.
As described above, there have been various types of microwave filter devices using conductor lines in the past. However, a common characteristic of all of them is the fact that their various lines are formed by patterning on two-dimensional planes on a single dielectric substrate. They are generally formed by etching on dielectric substrates (printed circuit boards) having copper foil surfaces.
However, these physical common structures used in the past actually have a large drawback. That is, filter devices of this type require a rather large space.
In concrete terms, let us assume, for example, that the conventional filter devices 10-50 illustrated in FIGS. 1-5 are designed for use with microwaves of the 1 GHz band. In this case, a quarter wavelength of the 1 GHz band will be at least 3 to 4 cm or longer. Thus, if for the sake of convenience the length of the filter device is made equivalent to the distance between the signal input terminal and output terminal and the device's width is made equivalent to the width in the direction orthogonal to that length, even the interdigital type BPF device 20 illustrated in FIG. 2 or the BRF device 50 illustrated in FIG. 5, which had the shortest lengths of those given thus far, will require a length of at least the order given above. The other filter devices will require lengths equal to about double or four times this length. That is, the required lengths will range from at least about 10 cm or less to nearly 20 cm at most.
Even the filters 20, 50 illustrated in FIGS. 2 and 5, which require lengths equivalent to only about a quarter wavelength, as described above, require additional area in the width direction to position the lines side by side, or similarly require dimensions equivalent to about a quarter of the wavelength for the resonator lines which are orthogonal to them.
Regions with such areas are quite large. For example, in electronic equipment handling microwaves such as radar detectors or satellite broadcast receivers, the substrates required for all circuits other than the filter devices require at the most dimensions of 10 cm or less even on their long sides. In comparison with them, one can understand what a large area is occupied by these conventional filter devices, which are only a single circuit element.
In actual fact, the size of the area occupied by these recent filter devices has been an extremely large obstacle to miniaturization of electronic circuit systems using them.
Incidentally, the interdigital type BPF device illustrated in FIG. 2, which is designed with a relatively small size, has a different drawback. That is, it requires additional patterning at the alternating ends of the input and output lines 11, 12 and the resonator lines 13, 14, or through-hole processing at numerous places, in order to give them electrical continuity with the grounding surface. This results in the inconvenience of increased complexity of the structure.
The following is another problem which is common to all the conventional microwave filter devices. That is, it is desirable to increase as much as possible the resonance coefficient Q of the dielectric substrates on which the strip lines are mounted, in order to suppress to a minimum the insertion loss of filters of this type. Nevertheless, in the actual products, which are electronic equipment, other elements must be taken into consideration when designing matters such as the materials of their substrates, rather than making their structural designs solely for the purpose of satisfying the electrical performance required in the filter devices. For this reason, there are restrictions on the substrate materials which can be used.
For example, in actual circuit designs, the printed circuit boards which serve also as the supporting substrates for the other peripheral circuit systems are substituted as the dielectric substrates of the conventional examples described above. That is, the strip line patterns which are needed for these filter devices are also formed at the same time as the conductive patterns for the other circuit elements are formed on the printed circuit boards.
Therefore, even though it is desirable for the filter devices themselves to use a substrate made of an expensive material such as Teflon which has a high Q value, nevertheless, it would be too wasteful, simply for the sake of the filter devices alone, to use such a high-quality material as Teflon in the substrate for the other circuit element parts as well. On the contrary, since the supreme requirement in general is reducing the costs, it was necessary to use materials which offered a tradeoff between the performance and costs of printed circuit boards. At the best, materials such as glass epoxy or paper phenol were used.